This invention relates to spool valve constructions for selectively directing pressurized fluid to fluid-operated devices and in one specific form relates to fluid pressure-actuated steering systems as employed in crawler tractor vehicles or the like.
Fluid systems which have control valves for selectively directing pressurized fluid to fluid pressure-actuated devices are usually supplied with operating fluid by a pump and usually have means for establishing a maximum pressure for the system. The pressure-limiting means is typically a primary relief valve connected between the output of the pump and the fluid tank or reservoir. In many instances the pump supplies fluid to a number of different devices through a number of control valves and one or more of the devices may require pressures which are lower than that established by the primary relief valve at least at certain stages of operation. It may be necessary that the pressure supplied to a particular device be modulatable or selectable within a range of pressures and it may be necessary to establish an upper limit for the range of pressures, supplied to that particular device, that is lower than the maximum system pressure established by the primary relief valve.
Heretofore a substantial amount of system complication, in the form of component complexity or additional components, has been required to provide for selective pressure modulation and for establishing a secondary relatively low maximum pressure limit for one or more specific devices. Such complications add significantly to bulk and cost of the system and increase maintenance requirements. It is desirable that these factors be minimized to the extent possible in fluid systems.
A specific example of a fluid system subject to the foregoing requirements and problems is the fluid pressure-operated steering systems used on crawler vehicles such as crawler tractors or the like. In vehicles of this kind each track assembly receives drive through a fluid pressure-actuated clutch and each track assembly is also provided with a fluid pressure-actuated brake. Turning of the vehicle is accomplished by depressing one of two foot pedals or the like to operate a control valve which first disengages the clutch and then engages the brake at the appropriate one of the two track assemblies. The resulting reduction or elimination of drive at one track assembly relative to the other accompanied by a selected degree of braking shifts the direction of travel of the vehicle.
In some instances the steering clutch may require actuating fluid at higher pressure than does the brake. Further, the brake fluid pressure should be modulatable as a function of foot pedal position to enable the operator to control the degree of braking. The pump which supplies fluid to the steering control valves may also serve other devices on the vehicle in addition to steering clutches, such as a transmission for example, which require a higher maximum operating pressure than the steering brakes. Secondary relief valves have commonly been utilized at the flow path between each steering control valve and track assembly brake to provide the lower maximum brake fluid pressure.
A closely related requirement in crawler vehicle steering systems of the above-described kind may be present as a result of the fact that manipulation of the operator's steering pedal simply shifts the spool of a control valve rather than directly applying pressure to a brake, as in other forms of vehicle brake system, in a way which would allow the operator to feel the degree of braking force which exists at various pedal positions. Consequently it is highly desirable in the interest of precision steering and ease of control to provide a form of feedback which increasingly resists foot pedal movement, as pedal depression is increased, with a force which is proportional to the degree of braking which is occurring at each instant. The providing of feedback of this kind has heretofore required still further substantial system complications.